REDDING, Calif., June 3 (Reuters) - Presumptive Republican U.S. presidential nominee Donald Trump on Friday denounced protesters in California as "thugs" a day after another demonstration outside one of his political rallies turned violent ahead of the state's presidential primary.

Demonstrators traded blows on Thursday evening in the street outside the San Jose Convention Center, videos posted to Twitter and online by media showed. Hundreds of protesters waved Mexican flags, chanted anti-Trump slogans and burned Trump hats and at least one U.S. flag.

Speaking before a packed crowd in the northern California city of Redding on Friday, Trump described the previous night's rally as "a love fest inside. No problems whatsoever." But then his supporters "walked out and they got accosted by a bunch of thugs," he said.

The protesters, many angry over Trump's rhetoric against illegal immigration, have gathered at Trump rallies for months. Trump, now the Republican Party's likely presidential nominee for the Nov. 8 election, canceled a rally in Chicago in March after clashes broke out between his supporters and protesters.

The San Jose Police Department reported 300 to 400 protesters had gathered outside the Trump rally on Thursday, where police formed lines to protect attendants exiting the convention center.

A number of the skirmishes occurred beyond police lines on nearby streets and at a parking garage, a Reuters photographer said. One sergeant suffered minor injuries after a protester struck him with a metal object. Police reported four arrests.

Over 250 officers staffed the "all-hands on deck event," said San Jose Police Chief Eddie Garcia.

"In hindsight, we'd say that wasn't enough," said Garcia. For future events of that scope, "we would need more officers with an absolute, number one goal of keeping both parties separate as much as we can."

San Jose Mayor Sam Liccardo, a Democrat, told the Associated Press that Trump needs to take responsibility for his supporters' conduct at the rallies.

But Trump, speaking in Redding on Friday, made light of the mayor's concerns.

"You know what I say when we have a protestor, which isn't very often, I say, 'Be very gentle, please don't hurt him ... If he punches you in the face, smile,'" Trump said.

Violence has peppered Trump's recent rallies in New Mexico and California, the U.S. state with the largest immigrant population, in advance of primary elections there on Tuesday.

The latest violence followed Democratic front-runner Hillary Clinton's scorching critique of Trump in a speech on Thursday in which she derided the real estate developer as a dangerous man with an angry, fearful world view.

Clinton told CNN on Friday that Trump had set a "low bar" regarding violence at poltical events. "Now is it a surprise that people who don't like him are stepping over that bar? I don't think it is," Clinton said.

But Friday afternoon's rally was peaceful as supporters from Redding and neighboring towns gathered as early as 8 a.m. local time (1500 GMT) to catch a glimpse of the candidate, many wearing hats emblazoned with Trump's signature slogan: "Make America Great Again."

Despite the unrest in San Jose, attendees said they felt at ease at Friday's event.

"We're both retired law enforcement," Heather Jimenez, a 45-year-old Cottonwood resident, said of herself and her husband. "No worries."

Although no formal protests broke out in Redding, the event attracted some who disapprove of Trump, but wanted to witness his high-flying campaign style in person.

"We've been hearing his nonsense," said Rachel Ochoa, a 57-year-old Redding resident who teaches English as a second language. "All we hear is him attacking the opponents and others."

Trump has accused Mexico of sending drug dealers and rapists across the U.S. border and has promised to build a wall between the two neighboring countries and make Mexico pay for it.

"We're gonna build that wall, folks," Trump said on Friday as the crowd chanted its support.

Trump also lamented the loss of American jobs to other countries, a key theme in his campaign. The message resonated in Redding, which residents say has been hit hard by the economic downturn.

"We've got a lot of people here out of work," said Joyce Tausch, a 78-year-old retiree who lives in Redding. "Trump is gonna do things for us."